The high frequency rectifier stage, usually the final stage of a switchmode power supply converter, contributes approximately 30% to 40% of the total loss of the converter. Traditionally, diode devices are used as rectifiers. Typical examples of such devices are silicon fast recovery rectifiers, Schottky rectifiers, GaAs ultra-fast recovery rectifiers, and fast recovery epitaxial diodes (FRED).
The diode can be represented as a device with a constant forward voltage drop in series with a dynamic resistance. This forward voltage drop often contributes a majority of the power dissipation of rectifier diodes.
As prior art power converter topology and semiconductor switching devices improved in performance, power converters in the range of 30-to 300 W became able to easily achieve 85% to 87% efficiency with conventional diode rectifiers. To further improve efficiency, it has become increasingly popular to use MOSFETs as rectifiers. The MOSFET source-drain has an intrinsic anti-parallel body diode that behaves like a moderate speed rectifier. When the MOSFET is off, the external circuit sees the body diode. When the MOSFET is driven on, the low turn-on channel resistance of the device is responsible for the conduction loss of the MOSFET, and the diode voltage drop disappears, resulting in lower power dissipation and therefore higher converter efficiency. A MOSFET used as a rectifier has to be driven on and off at the right time, hence the name Synchronous Rectifier.
Some power converter topologies are inherently suited for synchronous rectifier commutation. These topologies include the common forward converter and the Hybridge converter. These topologies are direct drive capable, i.e., the voltage that appears across the transformer secondary is a natural match of the current waveform. The body diode conducts before the correct gate drive voltage is asserted.
These simple circuits do, however, have certain limitations:
1) The transformer voltage has to be at the correct level, which is usually +/-20V for non-logic level MOSFETs and +/-10V maximum for logic level MOSFETs. Forcing in an appropriate drive level will result in excessive power dissipation and/or possible reduction in device reliability. If the voltage is not within an acceptable range, extra secondary windings or extra circuits have to be used to derive the gate drive signal.
2) The secondary voltage at different polarities has to be well defined under full line and load conditions, which implies that the transformer primary has to be suitably clamped to achieve such conditions. Workable examples of suitable topologies are clamped-forward, resonant reset forward, half-bridge and full-bridge topologies. Topologies with a simple transformer reset like a resistor, capacitor and diode network (RCD) snubbed forward converter are not synchronous rectifier friendly.
3) The transformer design has to give special consideration to the high gate-source capacitance of the MOSFETs used. Most of the time MOSFETs are connected in parallel to obtain low enough drain-source resistance, and therefore the gate-source is high enough to create a serious voltage spike problem on the transformer secondary. This makes the specification difficult, and causes gate drive signal deterioration.
4) Prior art circuits do not address reverse recovery, a major cause of synchronous power loss.
The above limitations of the prior art can be overcome by various topologies, but the topologies suitable for this purpose are generally unfriendly to synchronous rectifier commutation. Essentially, synchronous rectifier unfriendly topologies have the following characteristics:
1) The voltage that appears on the transformer secondary cannot provide the right timing or voltage level, even with complicated signal conditioning circuits. This results in cross-conduction and excessive power loss in the synchronous rectifier circuit.
2) Some topologies may work with a primary-derived synchronous rectifier drive signal; however, the complications of component count, safety isolation requirements and extra circuit power dissipation defeat the purpose of synchronous rectification.
3) The current profile of the secondary circuit creates excessive reverse recovery loss in the MOSFETs and excessive voltage spikes during turn-off. This makes synchronous rectifier applications ineffective.